A member of short-chain dehydrogenase/reductase family (sdr) is 17beta-hydroxy-steroid dehydrogenase (type VI), consisting of 309 amino acid residues, and containing a common characteristic conserved sequence of the sdr family. It has 39% homology at the amino acid level with another member of the family, 17beta-hydroxy-steroid dehydrogenas, (type III).
The enzyme 17beta-hydroxy-steroid dehydrogenase catalyzes the dehydrogenation/reduction of a special 17beta-hydroxy-steroid hormone or prohormone. The product can be used as a hormone to act directly or as a prohormone to be farther used by other enzymes. 17beta-hydroxy-steroid dehydrogenase (type I) reduces estrogen and androgen; 17beta-hydroxy-steroid dehydrogenase (type II) catalyzes the interconverter between testosterone and androstenedione and the interconversion between estradiol and estrone; 17beta-hydroxy-steroid dehydrogenase (type III) catalyzes redox reaction of androstenedione, testosterone and dihydrotestestosterone; 17beta-hydroxy-steroid dehydrogenase (type IV) catalyzes the interconversion between estradiol and estrone.
It is proved that 17beta-hydroxy-steroid dehydrogenase has numerous biological functions. Mutations in the 17beta-hydroxy-steroid dehydrogenase (type II) gene are possible causes of hereditary mammary-ovarian cancer. The enzyme plays an important role in the carcinogenesis of mammary gland and ovary in human. 17beta-hydroxy-steroid dehydrogenase (type III) plays an important role in the development of male sexuality. If this enzyme is in a low level resulted from genetic mutation, it may cause male pseudohermaphroditism and gynecomasty; 17beta-hydroxy-steroid dehydrogenase (type IV) plays an important role in catalyzing the oxidation of the precursor of bile acid. The common function of this hydroxy-steroid dehydrogenase family is to catalyze metabolism of steroid. Its product as a hormone binds with the receptor more efficiently than the prohormone thereby effecting functional rgulations. The product can also be used by other enzymes as a prohormone, thus continuing the pathway of steroid metabolism.